1. Field of the Invention
This invention relates generally to a turbine frame having a simplified structure and particularly relates to a turbine frame assembly incorporating a spoked array of bolts and spacer struts for rigidly interconnecting inner and outer concentrically aligned frame members while supporting a thermally unconstrained heat shield assembly.
2. Description of Prior Developments
Aircraft turbine engines employ two or more structural assemblies known as frames to support and accurately position the engine rotor within the stator. Each frame includes a stationary inner casing supported within a stationary outer casing by a number of radial struts contoured for minimum interference with the engine flow. The frame at the rear of the engine, usually aft of the turbine, is typically protected from the extreme turbine discharge gas temperatures by air cooled heat shields which include flowpath liners and fairings.
The construction of prior turbine engine frames has required the formation of numerous welded joints between the outer casing, the struts, the inner casing and the heat shield. Although the resulting welded frames performed satisfactorily, they were permanently fixed configurations which did not readily accommodate the replacement or repair of individual frame components. Moreover, such welded frames were often heavy and relatively difficult to construct. In addition, the resulting frame assemblies typically required replacement or repair after moderate periods of operation. These characteristics have posed significant operational and maintenance problems in aircraft applications where simplified repair, reduced weight and extended life are most desirable.
The heat shields which protect the inner and out casings are known as flowpath liners and together with the fairings thermally shield the entire rear engine frame. Heat shields are necessary to protect the engine frame because of the limited heat tolerance of available frame construction materials. Heat shields are also used to limit the thermal expansion and distortion of the frames. Excessive expansion and distortion of the frames caused by thermal gradients adversely affects the alignment of the rotor within the engine thereby adversely affecting engine performance.
As the hot gasses exiting the combustion and turbine section of an advanced gas turbine engine can be above the melting temperature of the available materials used in heat shield construction, the heat shields themselves must be efficiently cooled. The more efficient the heat shield cooling system is, the less cooling air is required to cool the heat shields and the, more efficient is the overall turbine engine cycle.
One of the most efficient methods of cooling the heat shields combines impingement cooling with film cooling. In this dual cooling method the cooling air first passes through a perforated plate known as an impingement baffle. The impingement baffle divides the cooling air into a multitude of small high velocity jets which impinge on and cool the back surface of the metal heat shields forming the flowpath liners. This portion of the cooling method is called impingement cooling.
The cooling air is then introduced into the engine flowpath through slots or holes known as air cooling film injection holes which extend through the flowpath liners. This creates a thermally protective film of cool air on the surfaces of the flowpath liners which are directly exposed to the hot exhaust gasses. This portion of the cooling method is called film cooling.
A long standing problem in designing heat shields for turbine engine frames has been the constraint of the thermal expansion of the hot flowpath heat shield surfaces caused by various structural members used to reinforce and secure the heat shields to the engine frames. The constraint of the thermal expansion of the heat shields has resulted in buckling and cracking of the heat shield surfaces and has imposed severe limits on the useful life of turbine frame heat shield systems incorporated within modern high performance engines.
Accordingly, a need exists for a lightweight, low cost turbine engine frame having an extended operational life and a relatively simple design which avoids the use of permanent weld joints. A need also exists for a turbine engine frame design which facilitates assembly and construction procedures, which obviates the use of special assembly tooling, and which facilitates replacement and repair of the frame members. A further need exists for a turbine engine frame which incorporates a free-floating heat shield adapted to thermally expand and contract virtually without constraint in order to minimize thermal stresses during engine operation.